I hear that with autoflowers one can leave the light on 24 hours, this is OK. What about Northern Lights, normally 18/6 until the flowering period, can I leave these with 24 hours of light until I want to induce flowering? Or will it be too much light? I know I need to go to 12/12 for flowering…but can leave them on 24 hour light on until then?
I would not personally. It is up to you. I do not experiment with my finer genetics. Some people will have you believe that you can veg all plants at 24/0; However, I am not one of them. 
18/6 max. - 14/10 min., during vegetative photo period. Shorter photo periods produce a higher % of females, and produce stronger root zones.
Autoflowers actually do need 24 hours of light indoors to maximize their potential, any less and they may flower much smaller than their full potential, although autos never really get very big and are ideal for indoor medical grows for people that don’t have a lot of room. Any tinkering with the light schedule and any shortening of the light period may result in the auto flowering right away at a very very small size.
All regular strains are usually grown with 18/6 for veg and 12/12 for flower. Yes regular photo period strains can be vegged under 24 hours of light, most of the time with no ill effects, however larger stronger root zones will be produced with some dark period. Being that cannabis is a C4 plant it does very little above ground during the night, this is why CO2 is not supplemented during dark periods and why a lot of fresh air is not as important during the dark period.
I have done it both ways with regular photo period plants and notice very little difference in the end product. 24 hours of light at the right time of veg, very early on in the development, can make for tighter internodes if this is desired. Then longer dark periods with some strains during flower, even longer than 12 hours of darkness can help some long flowering strains finish sooner.
All of these factors along with maybe saving some money on the electric bill will determine what light schedules you may wish to apply.